Seismic arrays consisting of source arrays and receiver arrays are used to study the strata of the rock structure below the ocean surface. To survey a specific area of the ocean floor, an exploration vessel is used to tow source and/or receiver arrays along a specified path covering the desired surface area of the ocean floor. The source and receiver arrays are used to obtain relevant geological data. Typically a single surface vessel will simultaneously tow both the source and receiver arrays, but sometimes an exploration vessel will only tow either the source array or receiver array. Other times, only a source array is towed while the receiver array is deployed on the ocean floor and remains stationary.
A prior art seismic receiver array is shown in FIG. 3 comprising multiple streamer cables typically ranging in length from approximately 1500 to 9000 meters. Each streamer cable is adapted with hydrophone groupings spaced at regular intervals along the length of the streamer cable. The streamer cables are deployed below the ocean surface at a specified depth or depths.
During operation, the acoustic shock wave generated by the source array penetrates the ocean floor and is reflected back. The reflected acoustic signatures are recorded by the hydrophone groupings in the streamer cables for subsequent analysis.
A seismic exploration vessel is generally capable of simultaneously towing both a source array and a receiver array. Importantly, lateral spreading forces must be applied to maintain the required spacing between the sub-arrays and streamer cables as they are towed behind the vessel. The required spacing depends, inter alia, on the number of streamer cables deployed and the interstitial spacing required between adjacent cables.
Typically, the air gun sub-arrays may be spread out laterally over a distance of approximately 12.5 to 100 meters, and the streamer cables over a distance of approximately 100 to 1500 meters.
A common method of achieving the required lateral spacing between sub-arrays and streamer cables is to deploy a divertor or paravane on a dedicated towing rope from the exploration vessel. A string of separation ropes are used to daisy-chain one streamer cable to the next to achieve the desired spacing between the streamer cables.
A number of approaches are employed to reduce the towing drag forces generated. Examples of such methods are the use of small diameter cables, high efficiency paravanes and the use of conventional cable fairings on separation ropes and lead-in cables. The prior art cable fairings utilize a single rope threaded through the nose of the fairing to allow the fairing to self-align with the direction of flow by way of free-swiveling.
Various methods of controlling the position of the towed equipment have been proposed to increase the quality of the seismic data being generated. One such method, for example, is the use of controllable foils or birds secured at approximately 300 meter intervals along the length of each streamer cable. The controllable foils are used to maintain the equipment at both a desired depth profile and a desired lateral offset throughout the length of the streamers.